1. Field of the Invention
The present invention belongs to the technical field of metallurgy, and particularly relates to a preparation method of oriented high silicon steel.
2. The Prior Arts
High silicon steel generally refers to Si—Fe alloy of which the silicon content is 4.5-6.5%, but 6.5% Si—Fe alloy has the characteristics of near zero magnetostriction Xs, high magnetic permeability, low coercivity, low iron loss, and especially low high-frequency iron loss, so that the 6.5% Si—Fe alloy becomes an ideal soft magnetic alloy material; however, when the Si content is increased to be 4.5% or more, alloy elongation sharply declines, and the elongation of the 6.5% Si—Fe alloy at room temperature is almost 0.
High silicon steel becomes a hot spot in the research of magnetic materials in recent years, and the research trends are mostly concentrated on formation laws of the ordered phase of non-oriented high silicon steel and trying to explain the causes and improvements on brittleness at room temperature; in Europe, Russia and Japan, there are reports that 6.5% Si non-oriented silicon steel is rolled out by adjusting alloy ingredients and optimizing the design of hot rolling-warm rolling-cold rolling procedure; Beijing University of Science and Technology improves the low-temperature plasticity of the strips by using B and other elements to refine cast structures and to prevent formation of DO3 long-range ordered phase (CN 1560309A). Japanese goes at the forefront in the in-depth research and industrialization sector of the high silicon steel: they carry out a comprehensive study about the improvement of forming properties of 6.5% Si by alloy elements Ni, Al, Mn, etc., and the influence of the adjustment of the rolling course on the forming performance, and propose an idea of obtaining a fiber texture through low temperature hot rolling so as to be convenient to lower temperature deformation (Takada Y, Journal of Applied Physics, 1988, 64, 5367-5369); some scholars even propose a thin strip quick quenching method to obtain high silicon microcrystalline strips which are 0.55 mm or even thinner so as to solve the brittleness problem (Arai K. I, Journal of Applied Physics, 1988, 64, 5373-5375).
Taken together, these methods for obtaining 6.5% Si electrical steel by composition and rolling deformation can solve the difficult problem for rolling forming of non-oriented high silicon steel to some extent, but with limited width and thickness, the microcrystalline thin strips prepared by the thin-strip quick quenching method are almost difficult to achieve industrial-scale production; it is Japanese NKK who truly promotes 6.5% Si non-oriented products to be practical: by using a chemical vapor deposition fast silicon infiltration (CVD) method and a rolling method, they produce 6.5% Si—Fe alloy of 0.1-0.5 mm (Haiji H. Journal of Magnetism and Magnetic Materials, 1996, 160, 109-114), known as “Super Ecore”; 3% Si non-oriented silicon electrical steel finished products are processed through silicon infiltration, then processed with high-temperature heat treatment for homogenization and promoted for grain growth to obtain 6.5% Si non-oriented electrical steel with favorable magnetic properties.
Because of single Goss texture obtained by secondary recrystallization, oriented silicon steel having superior magnetic properties of high magnetic induction and low iron loss in the rolling direction is mainly used for cores of various transformers; according to the conventional oriented silicon steel, the Si content is 2.8-3.4%, the Goss monocrystalline theoretical saturation magnetic induction with the Si content BS is about 2.03 T, and the value of B8 can directly reflect saturation magnetic induction of the oriented silicon steel sheet; according to the Hi-B (high magnetic induction) oriented silicon steel, B8 is between 1.90 and 1.96 T, B8/BS is greater than or equal to 0.936 and smaller than or equal to 0.966, and therefore, the Hi-B (high magnetic induction) oriented silicon steel is the highest level of products in oriented silicon steel.
The oriented high silicon steel has higher maximum permeability, higher resistivity, and lower high-frequency core loss, so that the mass and the volume of electrical components can be significantly reduced, the efficiency of electric appliances is improved, especially for the 6.5% Si—Fe alloy (saturation magnetic induction Bm is equal to 1.80 T), the magnetostriction is almost equal to 0, the noise of high-frequency transformers can be significantly reduced, and the oriented high silicon steel has a very high application value; however, for the preparation of oriented high silicon steel, we also need to solve a large number of technology problems, on one hand, both oriented high silicon steel and non-oriented high silicon steel need to solve the problem of matrix plasticity. On the other hand, the occurrence of complete secondary recrystallization of high silicon steel requires more stringent inhibitor conditions, so that the following factors clearly affect the preparation of the oriented high silicon steel:
1) an Si element can significantly improve the grain boundary migration of Fe—Si alloy and coarsen grains, so that high Si steel billets have a very coarse grain size, reach the level of tens of mm, and are unfavorable for plasticity;
2) the necessary condition for the secondary recrystallization is that the primarily recrystallized grain growth of the steel strips is strongly inhibited, and the increased grain boundary migration rate of cold-rolled high Si steel needs stronger inhibitors; and
3) the inhibitor can be a compound (such as an S compound and an N compound) or a simple substance (such as Cu, Sn, and B), but the former needs to be controlled by high temperature solution and phase change precipitation; billets heated at high temperature can cause crystalline grains to be too roughened; since the high silicon steel is a single phase ferrite, there is no phase transition window to control fine precipitation of the N compound. The simple substance and the compound are often used as auxiliary inhibitors, and when being used alone, the simple substance and the compound have insufficient restraining force, and also easily perform solution strengthening on the matrix, thus affecting plasticity.
Only a few Japanese patents give some reports about preparing the oriented high silicon steel by a conventional procedure: in Sumitomo Metal's patents JP S 63-069917 and 089622, billets of which the thicknesses are 50 mm are subjected to hot rolling-warm rolling-cold rolling to obtain 0.2-0.3 mm strips, and a single MnS, AlN, TiC or VC is used as an inhibitor to obtain 6.5% Si oriented silicon steel, but due to insufficient inhibition force of the inhibitor, secondary recrystallization has a low orientation level, and B8/BS=1.65 T/1.80 T=0.916; Nippon Steel Corporation increases the amount of AlN by a nitriding method after recognizing the problem of insufficient restraining force, but only enhances the magnetic induction B8 to 1.67 T (JP HO 4-080321,224625); obviously, these two methods do not break through the constraints of a conventional procedure.
In addition, the silicon infiltration method is also problematic when being used for preparing high-silica-oriented silicon steel: as mentioned above, the diffusion annealing course after a large amount of non-oriented Si is infiltrated in the steel promotes the growth of the crystalline grains, and such grain boundary migration in oriented silicon steel results in the reduction of the degree of orientation, even destroys the original sound secondary recrystallization, and finally cannot get good magnetic properties. There are no published reports in the research results of the current study about magnetic induction.
In twin-roll thin strip casting technique, two rotating casting rolls are used as crystallizers, and liquid molten steel is directly poured into a molten pool formed by the casting rolls and side block panels, and then directly solidified into thin strips of which the thickness is 1-6 mm, without casting, heating, hot-rolling, normalizing and other production working procedures. This technology is characterized in that the liquid metal is crystallized and solidified while undergoing pressure processing and plastic deformation, to complete the whole course conversion from liquid metal to solid thin strips in a very short period of time, at the solidification rate up to 102-104 DEG C./s, thus greatly refining the size of solidified crystalline grains of high silicon steel. Therefore, thin strip casting has a unique advantage in the production of high silicon Fe—Si alloy; in the respect, Sumitomo Metal Japan has related patent reports: they process 1-2 mm thin strips with casting-high temperature annealing-cold rolling to obtain strong Goss secondary recrystallization tissue; however, their recognition about the thin strip casting is limited, so that the yield of casting strips through direct cold rolling is low besides, the inhibition force of the inhibitors is weaker, and the oriented high silicon steel with superior magnetic induction is not obtained.